<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 200px;
            height: 50px;
            background-color: yellow;
            border: 1px solid gray;
        }
        div ul{
            /* display: none; */
            margin-top: 50px;
            transition: all 1s;
            height: 0px;
            overflow: hidden;
        }
        div:hover ul{
            /* display: block; */
            height: 200px;
        }
    </style>
</head>
<body>
    <div>我的菜单
        <ul>
            <li>1111</li>
            <li>2222</li>
            <li>3333</li>
            <li>4444</li>
            <li>5555</li>
        </ul>
    </div>
</body>
</html>